Although effective utilization of food resources, particularly protein, is in demand in view of the increasing population, the currently available protein is not always effectively utilized owing to disagreeable flavor. For example, it is well known that limited utilization of soybean protein preparations as a food ingredient is due to objectionable green bean flavor which is mainly brought about by aldehydes such as n-hexanal, propanal, 2,4-decadienal and the like. Since these aldehydes have very low threshold values, the presence of even a very small amount thereof can be perceived. Hence complete removal of the flavor components or complete conversion to compounds having much greater threshold values is necessary. These aldehydes, however, are present not only in free form but also in bound form in which the aldehydes interact so strongly with the soybean protein that no satisfactory method for complete removal thereof has heretofore been found.
As one of the known conversion methods, Japanese Patent Publication No. 9210/70 discloses a process for removing beany flavor from soybean milk by adding conidiospores of Aspergillus oryzae. In this method, n-hexanal in the soybean milk appears to be converted to n-hexanol which has a larger threshold value than n-hexanal. It has also been suggested, J. Agr. Food Chem. 23, 126(1975), that one can considerably reduce objectionable flavor of aldehydes through conversion of the aldehydes to alcohols by adding alcohol dehydrogenase and NADH. Although the threshold values of alcohols so produced are greater than the aldehydes, the alcohols nevertheless give off objectionable flavor under certain conditions. Moreover, in such process it is not certain that the aldehydes bound to proteins have been completely converted to the corresponding alcohols since the protein preparations containing such bound aldehydes regain objectionable flavor gradually during storage. It seems that such liberation is one of the causes for reversion flavor.